The present invention relates to an optical disk apparatus for reading out information recorded in an optical disk, in the form of a pit stream. More particularly, the invention relates to an optical disk apparatus which can read out information from both a high-density optical disk and a low-density optical disk, without necessity of using special optical elements.
Read-only optical disks for use as storage devices in computers or as media for recording audio and video signals have advanced in terms of their storage densities, from CD-ROMs to DVD-ROMs. A DVD-ROM is an optical disk based on the DVD-ROM standards. Various DVD-ROM drives have been developed to read out information from DVD-ROMs. The DVD-ROM drives have backward compatibility. In other words, each DVD drive can read out information not only from DVD-ROMs but also from CD-ROMs of older media. More precisely, it has CD-ROM readout compatibility, encouraging users to use newer media.
At present, so-called xe2x80x9chigh-definition DVDsxe2x80x9d are being developed. So are high-definition DVD drives for reading out information from high-definition DVDs. A high-definition DVD has a higher recording density than a DVD-ROM. It is important for a high-definition DVD drive to have DVD-ROM readout compatibility, as the DVD drive has CD-ROM readout compatibility. That is, the high-definition DVD drive must be designed to read out information also from the older optical disks, i.e., DVD-ROMs, so that users can use not only the information recorded on high-definition DVDs but also the information recorded on the DVD-ROMs.
To enhance the recording density of an optical disk is based on reducing the diameter of a beam spot for reading out information from the optical disk. The diameter of the beam spot is proportional to the wavelength of the light beam emitted from the light source used. The diameter is inversely proportional to the numerical aperture (NA) of the objective lens which focuses the light beam on the surface of the optical disk. To increase the recording density of an optical disk, from the density of a CD-ROM to that of a DVD-ROM, the wavelength of the light beam emitted from the light source has been decreased from 780 nm to 650 nm, and the NA of the objective lens has been increased from 0.45 to 0.6. As a significant change between a CD-ROM and DVD-ROM, the substrate thickness of DVD-ROMs has been reduced from 1.2 mm to 0.6 mm.
DVD drives having CD-ROM readout compatibility can be classified into three types, in accordance with the number of light sources used and the number of objective lenses used. The first type has two light sources and one objective lens. The second type has one light source and two objective lenses. The third type has two light sources and two objective lenses. These three types of DVD drive certainly costs more than the type of one light source and one objective lens.
To reduce cost, another type of a DVD drive has been proposed which comprises one light source, one objective lens, and an aperture limitation element or a holographic optical element (HOE). The light source and the objective lens are used to read out information from both a DVD-ROM and a CD-ROM. The aperture limitation element or HOE is provided to correcting the aberration resulting from the difference in thickness between the DVD-ROM and the CD-ROM.
A high-definition DVD drive must have an extra optical element (either an aperture limitation element or a HOE) in order to have DVD-ROM readout compatibility, if the high-definition DVD substrate is thinner than a DVD-ROM. (This situation is similar to the DVD drive which needs to have such an extra optical element to correct aberration resulting from the difference in substrate thickness between the DVD-ROM and the CD-ROM.)
If the high-definition DVD has the same thickness as the DVD-ROM, the high-definition DVD drive can have DVD-ROM readout compatibility, without necessity of having such an extra optical element. In this case, an information-readout beam can form a beam spot on the recording surface of a DVD-ROM without causing a great aberration because the high-definition DVD has the same thickness as the DVD-ROM.
Even though the information-readout beam forms a beam spot on the recording surface of the DVD-ROM without causing aberration, however, high-quality signals may not be read out from the DVD-ROM if the pit depth of the DVD-ROM is inappropriate for the wavelength of the information-readout beam or if the pit width of the DVD-ROM is excessive with respect to the diameter of the beam spot. If the pit depth is inappropriate, the signals will have an insufficient amplitude. If the width is excessive with respect to the diameter of the beam spot, so-called xe2x80x9creboundxe2x80x9d will occur in the signals corresponding to beams reflected from the long pits. These problems may be solved by use of particular optical elements. Use of such optical elements inevitably increases the manufacturing cost of the high-definition DVD drive. In view of this, it is desirable not to incorporate such optical elements in the high-definition DVD drive.
As mentioned above, a high-density optical disk drive (e.g., high-definition DVD drive) can form a beam spot on the recording surface of a low-density optical disk (e.g., DVD-ROM) without causing aberration, thereby to read out information from the low-density optical disk, even if it has no special optical elements, provided that the low-density optical disk has the same thickness as the high-density optical disk (e.g., high-definition DVD). However, if the pit depth is inappropriate for the wavelength of the light source, the signals will have an insufficient amplitude. If the pit width is excessive with respect to the diameter of the beam spot, the signals corresponding to beams reflected from the long pits will have an insufficient amplitude with xe2x80x9creboundxe2x80x9d. In either case, the high-density optical disk drive can hardly read out signals from the low-density optical disk.
The object of the present invention is to provide an optical disk apparatus for reading out information from a high-density optical disk, which has a light source for emitting a light beam and which can reproduce high-quality signals from a low-density optical disk, too, without using special optical elements for achieving backward compatibility, though the low density optical disk is designed for a longer wavelength than the one of the light beam.
According to the invention, there is provided an optical disk apparatus which comprises a light source for emitting a light beam, an optical system for focusing the light beam applied from the light source, thereby forming a beam spot on an optical disk in which information is recorded in the form of a pit stream, a tracking unit for detecting light reflected from the optical disk, generating a tracking error signal from the light by a Differential-Phase-Detection method (DPD method), and moving the beam spot in accordance with the tracking error signal, a spot-shifting section for shifting a center of the beam spot from a centerline of the pit stream by a predetermined distance in a radial direction of the optical disk, and an information-readout section for reading out the information from the optical disk.
According to the invention, there is provided an optical disk apparatus which comprises a light source for emitting a light beam, an optical system for focusing the light beam applied from the light source to form a beam spot on an optical disk in which information is recorded in the form of a pit stream, a quadrant photodetector having four detecting regions arranged in two rows and two columns for detecting light reflected from the optical disk, a delay circuit for delaying one of first and second sum signals, the first sum signal obtained from two of the detecting regions arranged in a diagonal and the second sum signal obtained from the remaining two of the detecting regions arranged in the other diagonal, a detecting section for detecting a phase difference between the first and second sum signals and generating a tracking error signal which is proportional to the phase difference detected, and a drive section for moving the beam spot in accordance with the tracking error signal generated by the detecting section to track the pit stream with the beam spot whose center is shifted from a centerline of the pit stream by a predetermined distance in a radial direction of the optical disk, and an information-readout section for reading out the information from the optical disk.
The predetermined distance can be set at any desired value by appropriately delaying the first or second sum signals.
According to the present invention, there is provided an optical disk apparatus comprising a disk-receiving section for receiving one of first and second optical disks which have a relatively high recording density and a relatively low recording density, respectively, and in each of which information is recorded in the form of pit streams, a light source for emitting a light beam, an optical system for focusing the light beam applied from the light source to form a beam spot on an optical disk, a tracking unit for detecting light reflected from the optical disk, generating a tracking error signal from the light by a DPD method, and moving the beam spot in accordance with the tracking error signal in a radial direction, a spot-shifting section for shifting a center of the beam spot from a centerline of the pit stream formed in the optical disk, by a predetermined distance in a radial direction of the optical disk, and an information-readout section for reading out the information from the optical disk. The spot-shifting section shifts a center of the beam spot from a centerline of the pit stream formed in the second optical disk, by a predetermined distance in a radial direction of the second optical disk when reading out information from the second optical disk.
When the beam spot is moved in the radial direction of the optical disk, with its center shifted from the centerline of the pit stream, it appears as if larger than it is. In this case, high-quality signals can be read out from the optical disk even if the pits have a width excessively large with respect to the beam spot. Further, when the beam spot is so moved, each pit appears shallower than it is, because the pit has a trapezoidal cross section. As a result, a signal having a sufficient amplitude can be reproduced even if the pit is excessively deep with respect to the wavelength of the light beam applied from the light source. The distance by which the beam spot must be moved should range from xc2xd to xc2xc of the top width of the pit, as measured in the radial direction of the optical disk.
The first optical disk is, for example, a high-definition DVD, whereas the second optical disk is, for example, an existing DVD-ROM, which is based on the DVD-ROM standards. The optical disk apparatus of the invention is therefore a high-definition DVD drive which has backward compatibility, capable of reading out information from a DVD-ROM as well. A DVD-ROM drive incorporates a light source which emits a light beam having a wavelength of 650 nm or 635 nm, not a light beam having a wave-length as short as 600 nm or less. On the other hand, the wavelength of a light source used in a high-definition DVD drive is considered to fall within a range of 400 to 450 nm. The high-definition DVD drive further comprises an objective lens having a numerical aperture (NA) of 0.6 or more as the objective lens incorporated in the DVD-ROM drive. Hence, the present invention is best applied to a high-definition DVD drive in which the following relation holds:
xcex/NA less than 1.0 (xcexcm). 
The optical disk apparatus according to the invention, for example, a high-definition DVD drive, has a light source for applying a light beam having a wavelength xcex to a high-density optical disk to reproduce information from the optical disk. The optical disk apparatus can reproduce high-quality signals from a low-density optical disk, too, without using special optical elements, though the low density optical disks are designed for a longer wavelength than the wavelength xcex. Hence, the apparatus can reproduce high-quality signals at a low cost, from not only the high-density optical disk but also the low-density optical disk.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.